You can either replay the same missions over and over or keep playing campaign missions. I got this while testing the achievement by replaying the first Spec-Ops sniper mission over and over. It's short and you kill about 10 or so enemies while protecting a team mate.
However this will come with time as you work on the game so no need to really grind this out.
Just one final note: This didn't seem to carry over from Win 8 to WP, I had to complete one more mission on the other platform from where I unlocked it first. So if you load up your WP after playing on Win 8 just play one more mission and this should unlock.